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Spherical scalar collapse in f (R) gravity is studied numerically in double-null coordinates in the Einstein frame. Dynamics in the vicinity of the singularity of the formed black hole is examined via mesh refinement and asymptotic analysis. Before the collapse, the scalar degree of freedom f is coupled to a physical scalar field, and general relativity is restored. During the collapse, the major energy of the physical scalar field moves to the center. As a result, f loses the coupling and becomes light, and gravity transits from general relativity to f (R) gravity. Due to strong gravity from the singularity and the low mass of f , f will cross the minimum of the potential and approach zero. Therefore, the dynamical solution is significantly different from the static solution of the black hole in f (R) gravity-it is not the de Sitter-Schwarzschild solution as one might have expected. f tries to suppress the evolution of the physical scalar field, which is a dark energy effect. As the singularity is approached, metric terms are dominant over other terms. The Kasner solution for spherical scalar collapse in f (R) theory is obtained and confirmed by numerical results. These results support the Belinskii-Khalatnikov-Lifshitz conjecture well.

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In this paper, we revisit the solar system tests of f (R) gravity. When the Sun sits in a vacuum, the field f is light, which leads to a metric different from the observations. We reobtain this result in a simpler way by directly focusing on the equations of motion for f (R) gravity in the Jordan frame. The discrepancy between the metric in the f (R) gravity and the observations can be alleviated by the chameleon mechanism. The implications from the chameleon mechanism on the functional form f (R) are discussed. Considering the analogy of the solar system tests to the false vacuum decay problem, the effective potentials in different cases are also explored. The combination of analytic and numerical approaches enables us to ascertain whether an f (R) model can pass the solar system tests or not.

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